Weird behaviour: A simple material renders fine on one mesh but not the other
There are Custom Split Normals on your triangle, set to (0,0,-1), by something. (An applied Edit Normal modifier?)
If there are shading anomalies, it's often worth checking what the renderer thinks the normals are, by displaying vertex-per-face normals in the Edit Mode 'Show Overlays' menu:

If you go to the triangle's Data tab &gt; Geometry Data panel, you can clear those.